Great evil has visited the United States but does that justify violent retaliation?

The Roman Catholic church, after all, is in the business of forgiveness and compassion.

We are all God's children and capable of redemption, even those who crash aircraft into buildings and those who mastermind their actions.

So how should they be punished?

It is a question preoccupying many governments and thinkers as the US gears up for an expected strike against Osama bin Laden and his Islamist network in Afghanistan, sending ethical, political and military considerations into collision.

John Paul II propelled himself into the heart of the debate by going ahead this week with a trip to Kazakhstan, a central Asian republic with a Muslim majority, and issuing a thinly-veiled message to the Bush administration.

To paraphrase, he said: you are entitled to attack the guilty but with restraint. The innocent must not suffer.

Vatican officials interpreted that as the green light for so-called "surgical strikes" against terrorist bases.

For a Pope who condemns capital punishment and railed against the US bombing of Iraq, that is a policy departure, of sorts, because he is accepting the justice of killing certain people.

He did not use those words and the overriding themes of his speeches was for peace between Christianity and Islam and the need to avert a global conflagration.

He switched from Russian to English for a pacifist prayer, to reach a global audience.

However, he also denounced terrorism in harsh language and called for those guilty of the September 11 attacks on New York and Washington to face justice.

His trusted spokesman, Joaquin Navarro-Valls, spelled out what that meant in an interview with the news agency Reuters.

"It is certain that, if someone has done great harm to society and there is a danger that if he remains free he may be able to do it again, you have the right to apply self-defence for the society which you lead, even though the means you may choose may be aggressive," he said.

Dr Navarro-Valls said the Pope understood that in defending a people and a nation it was sometimes more harmful to sit and do nothing.

"Sometimes it is more prudent to act rather than to be passive. In this sense, the Pope is not a pacifist because one must remember that in the name of peace even some horrible injustices can be carried out.

"Sometimes self defence implies an action which may lead to the death of a person.

"Either people who have carried out a horrendous crime are put in a position where they can do no further harm, by being handed over and put into custody, or the principle of self-defence applies with all its consequences.

"Christian ethics say that, when force is used as a last resort for self-defence, it must be proportionate to the threat and innocent people should not be harmed."

Peace sometimes had to take a back seat to self-defence, he said.

"In Christian ethics peace is a very high value, but the common good, both moral and physical, is sometimes above it.

"Some people in Europe would like to present the Pope as a pacifist and some people in America would like to see him as someone who wants to see the application of justice by any means. Both are wrong.

"Pacifism is a container where you could put almost anything. On the other hand, justice is something to be pursued, but at the same time seeing to it that by carrying out justice you don't inflict another injustice," said Dr Navarro Valls.

As tightropes go it was very thin and the spokesman knew it but the Pope was drawing on the principles of St Thomas Aquinas, the 13th century theologian, who said war must be a last resort, sanctioned by a legitimate authority and fought to redress an injury with peace being the ultimate goal.

Aquinas also stressed that civilian casualties were justified only if they were the unavoidable result of destroying a military target.

Yet for some Catholics, familiar with the Pope denouncing bloodshed, his guarded approval of military action came as a surprise.

He knows well that "surgical strikes" in Iraq brought painful deaths to women and children and could well do so in Afghanistan.

The Catholic church, just like any government and society, is divided over the correct response to the terrorist threat.

Several left-leaning priests in Italy have openly disagreed with John Paul and held vigils outside the US embassy, praying for no retaliation.

But some US bishops have virtually blessed the notion of American retaliation.